Buggy code:
public class ThreadGroup extends AbstractThreadGroup { private static final long serialVersionUID = 280L ; private static final Logger log = LoggingManager . getLoggerForClass ( ) ; private static final long WAIT_TO_DIE = JMeterUtils . getPropDefault ( "jmeterengine.threadstop.wait" , ( 5 * 1000 ) ) ; private static final int RAMPUP_GRANULARITY = JMeterUtils . getPropDefault ( "jmeterthread.rampup.granularity" , 1000 ) ; public static final String RAMP_TIME = "ThreadGroup.ramp_time" ; public static final String DELAYED_START = "ThreadGroup.delayedStart" ; public static final String SCHEDULER = "ThreadGroup.scheduler" ; public static final String START_TIME = "ThreadGroup.start_time" ; public static final String END_TIME = "ThreadGroup.end_time" ; public static final String DURATION = "ThreadGroup.duration" ; public static final String DELAY = "ThreadGroup.delay" ; private transient Thread threadStarter ; private Map < JMeterThread , Thread > allThreads = new ConcurrentHashMap < JMeterThread , Thread > ( ) ; private volatile boolean running = false ; private boolean delayedStartup ; public ThreadGroup ( ) { } public void setScheduler ( boolean Scheduler ) { } public boolean getScheduler ( ) { } public void setStartTime ( long stime ) { } public long getStartTime ( ) { } public long getDuration ( ) { } public void setDuration ( long duration ) { } public long getDelay ( ) { } public void setDelay ( long delay ) { } public void setEndTime ( long etime ) { } public long getEndTime ( ) { } public void setRampUp ( int rampUp ) { } public int getRampUp ( ) { } private boolean isDelayedStartup ( ) { } private void scheduleThread ( JMeterThread thread , long now ) { } private void delayBy ( long delay ) { } @ Override public void start ( int groupCount , ListenerNotifier notifier , ListedHashTree threadGroupTree , StandardJMeterEngine engine ) { } private void registerStartedThread ( JMeterThread jMeterThread , Thread newThread ) { } private JMeterThread makeThread ( int groupCount , ListenerNotifier notifier , ListedHashTree threadGroupTree , StandardJMeterEngine engine , int numThreads , int i , JMeterContext context ) { } @ Override public boolean stopThread ( String threadName , boolean now ) { } public void threadFinished ( JMeterThread thread ) { <START_BUG> ThreadGroup . log . info ( ( "Ending<seq2seq4repair_space>thread<seq2seq4repair_space>" + ( thread . getThreadName ( ) ) ) ) ; <END_BUG> allThreads . remove ( thread ) ; } @ Override public void tellThreadsToStop ( ) { } @ Override public void stop ( ) { } @ Override public int numberOfActiveThreads ( ) { } @ Override public boolean verifyThreadsStopped ( ) { } private boolean verifyThreadStopped ( Thread thread ) { } @ Override public void waitThreadsStopped ( ) { } private void waitThreadStopped ( Thread thread ) { } private ListedHashTree cloneTree ( ListedHashTree tree ) { } private void pause ( long ms ) { } class ThreadStarter implements Runnable { private final int groupCount ; private final ListenerNotifier notifier ; private final ListedHashTree threadGroupTree ; private final StandardJMeterEngine engine ; private final JMeterContext context ; public ThreadStarter ( int groupCount , ListenerNotifier notifier , ListedHashTree threadGroupTree , StandardJMeterEngine engine ) { } public void run ( ) { } } }

Fix:
ThreadGroup . log . debug ( ( "Ending<seq2seq4repair_space>thread<seq2seq4repair_space>" + ( thread . getThreadName ( ) ) ) ) ;